Rate of Reactions

Reaction rate measures how quickly a reactant is used up or a product forms. Reacting particles must collide, and collisions must have enough energy to lead to reaction. An explanation should connect the changed condition to particle behaviour, then to the number of successful collisions per unit time.

A larger lump and equal-mass powder can produce the same final amount of gas while reacting at different speeds. In contrast, adding more of a limiting reactant can change the final amount. Keep the question "how fast?" separate from "how much?".

For a gas syringe, check airtight connections and free plunger movement. For mass loss, prevent splashes so that lost liquid is not mistaken for escaped gas. Start timing consistently. A subjective colour or visibility endpoint is less precise than a clear instrumental measurement, so use the same criterion and repeat.

The gradient of a product-volume graph gives rate. A steeper initial slope means faster initial reaction. The curve becomes less steep as reactants are consumed. A horizontal line means no further gas is collected; a reactant may have been exhausted. A tangent estimates the instantaneous rate at a point; a line joining two measured points gives the average over that interval.

For a mass-loss graph, the mass decreases, so the slope is negative. Use the magnitude of the decrease per time for a positive reaction-rate value. Compare final gas volumes only at the same temperature and pressure. A leak can reduce apparent volume without changing the actual chemical yield.

A catalyst increases reaction rate and is chemically unchanged overall at the end. It can take part in intermediate steps before being regenerated. It provides an alternative pathway with lower activation energy, so a larger fraction of collisions can succeed at the same temperature. It does not supply the reaction energy or increase the final yield from a fixed limiting reactant.

On an energy profile, the catalysed route has a lower peak but the same reactant and product levels. Enzymes also increase rates, but their activity depends on conditions; heating them too much can change their structure and reduce activity. Do not treat every catalyst as able to work under any conditions.

To investigate particle size instead, vary chip/powder size while holding carbonate mass and acid concentration/volume constant. Use acid in excess if you want final gas amount fixed by carbonate. A suitable method controls the variables that could give the same observed effect, not just the easiest variables to measure.

Rate describes change per unit time. Concentration, pressure, temperature and exposed surface area affect successful collisions. On a progress graph, slope indicates rate and the final level indicates measured amount.